Green flash

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Green flashes and green rays are rare optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible for a short period of time above the sun, or a green ray shoots up from the sunset point. It is usually observed from a low altitude where there is an unobstructed view of the horizon, such as on the ocean.

Green flash observed at the La Silla Observatory on October 15, 2005

Green flash observed in San Francisco on September 17, 2006

1 Explanation
2 Types of green flashes
- 2.1 Blue flashes
3 In the media
4 External links

[edit] Explanation

Its explanation lies in refraction of light (as in a prism) in the atmosphere and is enhanced by atmospheric inversions. Whilst we would expect to see a blue light, the blue is dispersed (this is why the sky is blue) and only the green light remains visible.

With slight magnification a green rim on the top limb of the solar disk can be seen on most clear-day sunsets. However the flash or ray effects require a stronger layering of the atmosphere and a mirage which serves to magnify the green for a fraction of a second to a couple of seconds.

Image of Green Flash

[edit] Types of green flashes

The green flash is actually a group of phenomena, some of which are listed below ([1]):

Phenomenon	Characteristics	Conditions	Best seen from
inferior-mirage flash	Joule's “last glimpse”; oval, flattened below; lasts 1 or 2 seconds	surface warmer than the overlying air	close to sea level
mock-mirage flash	indentations seem to “pinch off” a thin, pointy strip from the upper limb of the sun; duration 1 or 2 sec.	atmospheric inversion layer below eye level; surface colder than air	the higher the eye, the more likely; flash is most obvious when the eye is just above the inversion
sub-duct flash	large upper part of an hourglass-shaped sun turns green for up to 15 sec.	observer below a strong atmospheric inversion	in a narrow height interval just below a duct (can occur at any height)
green ray	green beam of light that seems to shoot up from a green flash, or is seen immediately after sundown. Usually only a few degrees long; lasts no longer than a couple of seconds	hazy air and a bright green flash of one of the kinds named above that acts as a light source	unknown

Most observations involve inferior-mirage flashes, while the rest are mostly mock-mirage flashes. The two other kinds are rare, constituting only about 1% of all reported flashes. The green flash phenomena that are not listed here - such as the cloud-top flash, which is usually seen as the sun sinks into the coastal fog and sometimes also at distant cumulus clouds — are omitted because they are not fully understood. Moreover, Alistair Fraser's flashes, which are seen in hilly country, are a variant of the mock-mirage flash in places where inversions are pushed up over hills and are subsequently not listed in the chart.

[edit] Blue flashes

Very occasionally, the amount of blue light is sufficient to be visible as a "blue flash". [2] (This is not to be confused with the blue flash reported to occur during nuclear criticality accidents.)

[edit] In the media

Jules Verne, the early French science-fiction writer, wrote a book named Le Rayon vert (The Green Ray, 1882) whose heroine (Helena Campbell) is chasing this elusive phenomenon. According to legend, those who see the ray will not choose wrong at love.
Éric Rohmer, a French film director, made a movie named The Green Ray (film). The green ray is visible in the last scene.
Carl Hiassen, an American author, wrote a novel called Flush, in which the characters see a green flash at the end of the book.
The green flash features a couple of times in the works of Arthur C. Clarke, UK science-fiction writer.
Raymond Feist, a fantasy writer, mentions it once to demonstrate one of the rare treasures of a life of traveling.
Sonic Youth's song "Jams Run Free" describes the occurrence in its chorus. chorus
The green flash is mentioned in Middlesex by Jeffrey Eugenides
There is a beautiful fragment (in Spanish) by Julio Cortazar, on the 'Green Ray', see [3]
Roger Penrose, in his book "The Road to Reality" Epilogue includes the 'Green Flash'
The Green Flash supposedly being used in Pirates of the Caribbean: At World's End to portray the means a soul has returned to the world of the living. This is a legend of pirates.